The present invention arises from inadequacies in previous techniques for presenting a mass of discrete particles for interaction with a medium. A specific example of these inadequacies lies in the field of respirators. One presently commercial face mask for removing noxious vapors from the air comprises a porous nonwoven sheet in which alumina particles are dispersed (the alumina particles are cascaded into a fluffy nonwoven web of staple fibers prepared by "rando-webbing" or garnetting, and the web is then compressed and cut into sheets of the desired shape, whereupon the edges of the cut sheets heat-seal together). While the mask works effectively to remove the noxious vapors, the life of the mask is shorter than desired.
The short life of this face mask has been traced to difficulties in providing and maintaining a uniform distribution of particles. It is difficult to initially obtain a uniform distribution of particles by cascading them into a fluffy nonwoven web of staple fibers. More than that, it is believed that particles within the completed sheet migrate through the interstices of the fibrous web as a result of normal handling or vibration of the mask or as a result of air flow through the mask. The result is that thin spots develop in the array of particles. Eventually a "breakthrough" of noxious vapors occurs at the thin spot, and the effective life of the mask is ended. While the weight of alumina particles could be increased to lengthen the life of the mask, such a change would also increase the static pressure of the mask (that is, the pressure drop through the mask), whereupon breathing through the mask would be more difficult.
The described technique for supporting particles for interaction with a medium is just one of many that have been proposed or used, but generally all of the previous approaches require some unsatisfactory compromise in properties. Some require an undesirably high static pressure or pressure drop (as in packed beds of the particles, which otherwise have maximum exposed surface area, or as when particles are impregnated into or coated onto fibrous papers; see U.S. Pat. Nos. 328,947 and 3,158,532). Some require too many ingredients besides the particles themselves (such as binder materials, fiber sizing agents, or other additives), which limits the utility of the products because of chemical or other characteristics of the added ingredients (see U.S. Pat. Nos. 2,369,462 and 3,745,060). Some require covering part of the reactive surface of the particles and therefore lessening the efficiency of the particles, as when binder material is used to adhere the particles in place in a web or to themselves (see U.S. Pat. Nos. 3,801,400; 3,745,060; 3,615,995; 2,988,469; and 3,474,600). And some require elaborate and expensive supporting apparatus, as for packed beds of the particles or for certain mixtures of fibers and particles (see U.S. Pat. No. 3,083,157). While each of the described approaches has its own uses and advantages, their inadequacies, including those listed above, leads to a need for a new, superior technique for supporting a mass of particles.